Besides the standard costs of owning a property such as maintenance, cleaning, repairs, utility, etc there are a few taxes and fees that have to be paid for by the owner.
Property Ownership Tax (IBI in Spanish), the owner, being a Spanish resident or not, has to pay this local tax. It is calculated on the basis of an administrative value (valor catastral in Spanish). The rate varies from 0,4% till 1,1% depending in which region and area the property is located in the town.
Annual Wealth Tax (Impuesto Sobre Patrimonio), fiscal or non-fiscal resident, since january 2008 there is a 100% discount so you should not have to declare this tax anymore. There are talks that from 2012 this tax will have to be paid again.
Personal Income Tax (IRNR, Impuesto de la Renta de No Residente), as a non-fiscal resident and owner of real estate in Spain you have to declare and pay this tax based on the administrative value of the purchase price. The rate is maximum 24,75% over 1,1% over 50% of the purchase price of the property.
Rental income, if you as a non resident do rent the property out, you will usually have to pay a flat rate of 24,75% of the gross income you earn from renting the property. As a fiscal resident the rental income will be taken into account when doing your annual tax return.
Community Fees, if you have a property that is part of a development or have communal zones with other property owners you are by law obliged to be a member of the community of owners (Comunidad de Propietarios). You will have to pay for the use of common areas and services the community wants. Each year there must be a general meeting where the budget for the next year must be presented and approved by the owners.
Insurance, the usual household insurance. The price will depend on the type of property and the amount that has to be insured. In case of an apartment the building will usually be insured by the community.